1. Technical Field
The present invention generally relates to an advantageous new field of optical devices based on resonant leaky modes in periodically modulated films. More particularly, the present invention relates to tunable optical devices based on known nano/micro-electromechanical (N/MEMS) actuation methods, and useful applications thereof.
2. Description of the Background Art
The field of thin-film optics is a mature area of optical technology. There are numerous companies and/or industries producing optical filters and devices of a great variety throughout the world. These devices typically consist of homogeneous layers deposited with precise thicknesses and materials parameters, often under vacuum. Examples of currently marketed devices utilizing multilayered arrangements include, but are not limited to, laser mirrors; polarizers; antireflection film systems; bandpass filters; bandstop filters; edge filters; lowpass filters; high-pass filters; phase plates; tunable optical devices and/or filters; sensors; modulators; polarization control devices; hyper-spectral arrays; sensor arrays; beam splitters; and others.
A significant drawback associated with current optical devices is that a large number of layers, for example, from 10 to 200, are often needed to fashion the spectral and angular properties required for a particular application. These optical devices, which utilize homogeneous layer stacks, operate on the basis of multiple reflections between the interfaces incorporated in a layer stack. Thus, the amount and cost of material required to effectuate a desired optical effect can be significant. In addition, the adhesion difficulties associated with forming the multilayered stacks can cause problems. Further, there are interface scattering losses inherently associated with multilayered arrangements.
Materials that are artificially structured on a nanoscale exhibit electronic and photonic properties that differ dramatically from those of the corresponding bulk entity. Periodic volumes and layers are expected to yield new devices in many fields including communications, medicine, and laser technology. [See, J. D. Joannopoulos, R. D. Meade, and J. N. Winn, Photonic Crystals Molding the Flow of Light, Princeton, 1995; A. Yariv and P. Yeh, Photonics: Optical Electronics in Modern Communications, 6th edition, Oxford University Press, New York, 2007; K. Sakoda, Optical Properties of Photonic Crystals, Springer-Verlag, Berlin, 2001.] On incorporation of mechanical mobility into nanostructured systems, further enhancements and advances in performance can be realized.
Subwavelength photonic lattices are presently of immense interest owing to their applicability in numerous optical systems and devices including biosensors, lasers, and filters. When the lattice is confined to a layer, thereby forming a periodic waveguide, an incident optical wave may undergo a guided-mode resonance (GMR) on coupling to a leaky eigenmode of the layer system. The external spectral signatures can have complex shapes with high efficiency in both reflection and transmission. It has been shown that subwavelength periodic leaky-mode waveguide films provide diverse spectral characteristics such that even single-layer elements can function as narrow-line bandpass filters, polarized wideband reflectors, wideband polarizers, polarization-independent elements, and wideband antireflection films. [See, R. Magnusson, Y. Ding, K. J. Lee, D. Shin, P. S. Priambodo, P. P. Young, and T. A. Maldonado, “Photonic devices enabled by waveguide-mode resonance effects in periodically modulated films,” in Nano- and Micro-Optics for Information Systems, L. A. Eldada, ed., Proc. SPIE 5225, 20-34 (2003); Y. Ding and R. Magnusson, “Use of nondegenerate resonant leaky modes to fashion diverse optical spectra,” Opt. Express, 12, 1885-1891 (2004); Y. Ding and R. Magnusson, “Resonant leaky-mode spectral-band engineering and device applications,” Opt. Express 12, 5661-5674 (2004); R. Magnusson and Y. Ding, “Spectral-band engineering with interacting resonant leaky modes in thin periodic films,” Proc. SPIE, Vol. 5720, Micromachining Technology for Microoptics and Nanooptics, pp. 119-129, Photonics West, January 2005.] The relevant physical properties of these elements can be explained in terms of the structure of the second (leaky) photonic stopband and its relation to the symmetry of the periodic profile.
The interaction dynamics of the leaky modes at resonance contribute to sculpting the spectral bands. The leaky-mode spectral placement, their spectral density, and their levels of interaction decisively affect device operation and associated functionality. [See, Y. Ding et al., “Use of nondegenerate resonant leaky modes to fashion diverse optical spectra,” Opt. Express, 12, 1885-1891 (2004); Y. Ding et al., “Resonant leaky-mode spectral-band engineering and device applications,” Opt. Express 12, 5661-5674 (2004).] Additional background information is set forth in the above-referenced provisional/non-provisional applications that are incorporated herein by reference. [See Cross-Reference to Related Applications.]
Guided-mode resonance device parameters, including refractive index of grating layers or surrounding media, thickness, period, and fill factor, can all be applied to implement tunability. In past publications, a tunable laser using a rotating resonance element (i.e., angular tuning) and a photorefractive tunable filter were described. [S. S. Wang and R. Magnusson, “Theory and applications of guided-mode resonance filters,” Appl. Opt., vol. 32, pp. 2606-2613, May 1993.] Furthermore, tuning can be accomplished by changing layer thickness or material refractive index, a method of significance in resonant sensor operation. Shu et al. reported analysis of a tunable structure consisting of two adjacent photonic-crystal films, each composing a two-dimensional waveguide grating that could be displaced laterally or longitudinally by a mechanical force. [W. Shu, M. F. Yanik, O. Solgaard, and S. Fan, “Displacement-sensitive photonic crystal structures based on guided resonances in photonic crystal slabs,” Appl. Phys. Lett., vol. 82, pp. 1999-2001, 31 Mar. 2003.] Each periodic waveguide admitted guided-mode resonances whose coupling could be mechanically altered for spectral tuning. Carr et al. theoretically studied laterally deformable nanomechanical gratings under resonance conditions and, using a fixed input wavelength, found intensity modulation and polarization effects on application of grating lateral shifts. [D. W. Carr, J. P. Sullivan, and T. A. Friedman, “Laterally deformable nanomechanical zeroth-order gratings: anomalous diffraction studied by rigorous coupled-wave theory,” Optics Letters 28, pp. 1636-1638 (2003).] Later, Keeler et al. fabricated a prototype device in amorphous diamond with 600 nm period and nanoscale features and suggested uses for inertial sensing and modulation [B. E. N. Keeler, D. W. Carr, J. P. Sullivan, T. A. Friedman, and J. R. Wendt, “Experimental demonstration of a laterally deformable optical nanoelectromechanical system grating transducer,” Optics Letters 29, pp. 1182-1184 (2004).]
More recently, Kanamori et al. fabricated two-dimensional (and thus polarization independent) GMR filters that could be moved, via micro-electromechanical (MEMS) actuators, within an air gap to add or drop selected wavelengths in a bit stream. The silicon-on-insulator device had about 80% efficiency at 1500 nm. [Y. Kanamori, T. Kitani, and K. Hane, “Movable Guided-Mode Resonant Grating Filters by Four Bimorph Actuators for Wavelength Selective Dynamic Reflection Control,” 2006 IEEE/LEOS International Conference on Optical MEMS and their applications, Big Sky, Mont., Aug. 21-24, 2006.] Huang et al. applied a mobile GMR mirror to tune a surface emitting laser by electrostatic actuation. The point was made there that this thin (230 nm) element is a significant advance over similar tuning with Bragg stacks that might be about 10 μm thick; the laser could be tuned across a 2.2 nm spectral range in the reported experiment. [Michael C. Y. Huang, Ye Zhou and Connie J. Chang-Hasnain, “Nanoscale High-Contrast Subwavelength Grating Integrated Tunable Surface Emitting Laser,” 2006 IEEE/LEOS International Conference on Optical MEMS and their applications, Big Sky, Mont., Aug. 21-24, 2006.] Finally, Magnusson et al. initial results on widely tunable SOI elements. [R. Magnusson and Y. Ding, “MEMS tunable resonant leaky mode filters,” IEEE Photonics Technology Letters, vol. 18, pp. 1479-1481, Jul. 15, 2006.]
Additionally, numerous other tunable grating structures, not inducing leaky modes, have been described in the literature. As an example of a device in this class, Nakagawa and Fainman presented a structure in which a subwavelength grating was placed between planar dielectric mirrors composing a Fabry-Perot cavity. [W. Nakagawa and Y. Fainman, “Tunable optical nanocavity based on modulation of near-field coupling between subwavelength periodic nanostructures,” IEEE J. Selected Topics in Quantum Electronics, vol. 10, pp. 478-483, May/June 2004.] Lateral and longitudinal motion yielded effective tuning via associated near-field coupling mechanisms. Park and Lee presented a tunable nanophotonic grating layer that was placed on a flexible substrate. [W. Park and J. B. Lee, “Mechanically tunable photonic crystal structures,” Appl. Phys. Lett., vol. 85, pp. 4845-4847, 22 Nov. 2004.] By mechanically stretching the lattice, thereby changing the grating period, a variation in the angle of refraction of an incident beam of light was achieved. Kim et al. applied surface plasmons and Rayleigh anomaly to obtain enhanced transmission through a metallic perforated film. They tuned the device by embedding it in a liquid crystal and then varying the surrounding refractive index by applied voltage. [Tae Jin Kim, Tineke Thio, T. W. Ebbesen, D. E. Grupp, and H. J. Lezec, “Control of optical transmission through metals perforated with subwavelength hole arrays,” Optics Letters 24, pp 256-258 (1999).].
The patent literature reflects previous developments involving optical devices. For example, U.S. Pat. No. 5,216,680 to Magnusson et al. describes a guided-mode resonance filter which can be used as an optical filter with narrow line width and as an efficient optical switch. Diffraction efficiencies and passband frequencies are calculated based on guided-mode resonance properties of periodic dielectric structures in a waveguide geometry. The guided-mode resonance filter preferably includes means for changing various parameters within the grating so as to change passband frequencies in response thereto. The disclosed diffraction grating could be supported by a semiconductor substrate adjacent to a semiconductor laser for fine-tuning the output of the semiconductor laser. The diffraction grating between thin-film layers of the Magnusson '680 patent can be designed so as to enhance the thin-film performance characteristics of the structure.
U.S. Pat. No. 5,598,300 to Magnusson et al. discloses ideal or near ideal filters for reflecting or transmitting electromagnetic waves having a wavelength range of approximately 100 nm to 10 cm. The disclosed filters combine a dielectric multilayer structure with a resonant waveguide grating and are valid for both TE and TM polarized waves. High efficiency, arbitrarily narrow band, polarized filters with low sidebands over extended wavelength ranges can be obtained according to the teachings of the Magnusson '300 patent. In addition, U.S. Pat. No. 6,154,480 to Magnusson et al. discloses vertical-cavity lasers (VCLs) fabricated without Bragg mirrors by replacing them with diffractive GMR mirrors with much fewer layers, for example, two or three layers. When incorporated in VCLs, the GMR mirrors yield single-mode, narrow-line, highly-polarized output light.
Notwithstanding that which is presently known with respect to optical devices and optical device-related technologies, a need remains for optical devices and optical device-related technologies that facilitate greater optical design and fabrication flexibility. In addition, a need remains for optical devices and optical device-related technologies that facilitate shaping of the reflection and transmission spectra of optical devices. A further need exists for mechanically tunable electromagnetic and photonic devices and, more particularly, for tunable structures that support leaky waveguide modes. A need also exists for structures that coexhibit plasmonic and leaky mode properties. Such concepts and devices have tremendous potential for beneficial uses.
These and other needs are met by the systems and methods disclosed herein. In addition, the disclosed systems and methods address numerous problems and shortcomings commonly associated with known optical devices and optical device-related technologies, thereby providing means for achieving greater optical design flexibility and effectiveness.